diff --git a/__tests__/util.test.ts b/__tests__/util.test.ts
index bb7560dc..d60c051c 100644
--- a/__tests__/util.test.ts
+++ b/__tests__/util.test.ts
@@ -3,7 +3,8 @@ import * as core from '@actions/core';
 import {
   convertVersionToSemver,
   isVersionSatisfies,
-  isCacheFeatureAvailable
+  isCacheFeatureAvailable,
+  isGhes
 } from '../src/util';
 
 jest.mock('@actions/cache');
@@ -80,3 +81,42 @@ describe('convertVersionToSemver', () => {
     expect(actual).toBe(expected);
   });
 });
+
+describe('isGhes', () => {
+  const pristineEnv = process.env;
+
+  beforeEach(() => {
+    jest.resetModules();
+    process.env = { ...pristineEnv };
+  });
+
+  afterAll(() => {
+    process.env = pristineEnv;
+  });  
+
+  it('returns false when the GITHUB_SERVER_URL environment variable is not defined', async () => {
+    process.env['GITHUB_SERVER_URL'] = undefined;
+    expect(isGhes()).toBeFalsy();
+  });
+
+  it('returns false when the GITHUB_SERVER_URL environment variable is set to github.com', async () => {
+    process.env['GITHUB_SERVER_URL'] = 'https://github.com';
+    expect(isGhes()).toBeFalsy();
+  });
+
+  it('returns false when the GITHUB_SERVER_URL environment variable is set to a GitHub Enterprise Cloud-style URL', async () => {
+    process.env['GITHUB_SERVER_URL'] = 'https://contoso.ghe.com';
+    expect(isGhes()).toBeFalsy();
+  });
+
+  it('returns false when the GITHUB_SERVER_URL environment variable has a .localhost suffix', async () => {
+    process.env['GITHUB_SERVER_URL'] = 'https://mock-github.localhost';
+    expect(isGhes()).toBeFalsy();
+  });
+
+  it('returns true when the GITHUB_SERVER_URL environment variable is set to some other URL', async () => {
+    process.env['GITHUB_SERVER_URL'] = 'https://src.onpremise.fabrikam.com';
+    expect(isGhes()).toBeTruthy();
+  });
+});
+